companion vs keep company

companion is a noun but keep company is not a noun.

Word	Noun	Adjective	Verb	Adverb
companion	Yes	No	Yes	No
keep company	No	No	Yes	No

As verbs, companion and keep company are synonyms defined as:

Other synonyms of companion include accompany, company.

companion (noun)	keep company (noun)
a friend who is frequently in the company of another
one paid to accompany or assist or live with another
a traveler who accompanies you

companion (verb)	keep company (verb)
be a companion to somebody	be a companion to somebody